scons: Fix build with llvm-12.

scons: Fix build with llvm-12.

This patch fixes these SCons build errors.

/usr/bin/ld: build/linux-x86_64-debug/gallium/auxiliary/libgallium.a(lp_bld_misc.os): in function `llvm::InitializeNativeTarget()':
llvm/Support/TargetSelect.h:118: undefined reference to `LLVMInitializeX86TargetInfo'
/usr/bin/ld: llvm/Support/TargetSelect.h:119: undefined reference to `LLVMInitializeX86Target'
/usr/bin/ld: llvm/Support/TargetSelect.h:120: undefined reference to `LLVMInitializeX86TargetMC'
/usr/bin/ld: build/linux-x86_64-debug/gallium/auxiliary/libgallium.a(lp_bld_misc.os): in function `llvm::InitializeNativeTargetAsmPrinter()':
llvm/Support/TargetSelect.h:132: undefined reference to `LLVMInitializeX86AsmPrinter'
/usr/bin/ld: build/linux-x86_64-debug/gallium/auxiliary/libgallium.a(lp_bld_misc.os): in function `llvm::InitializeNativeTargetDisassembler()':
llvm/Support/TargetSelect.h:156: undefined reference to `LLVMInitializeX86Disassembler'

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7722>